1. Technical Field
The present disclosure relates to a method for manufacturing electronic devices integrated on a semiconductor substrate, in particular silicon carbide, and, more specifically, to a method for manufacturing electronic devices on a semiconductor substrate with wide band gap. The disclosure also relates to a vertical conduction power electronic MOSFET device integrated on a semiconductor substrate with wide band gap.
The disclosure also make reference to a power electronic MOSFET device integrated on a semiconductor substrate with wide band gap comprising at least a first implanted region of a first type of conductivity, at least a second implanted region of a second type of conductivity formed inside said at least a first implanted region, a gate region projecting from said substrate and insulated therefrom by means of a dielectric layer.
The disclosure particularly, but not exclusively, relates to a method for manufacturing power MOS transistors and the following description is made with reference to this field of application by way of explanation only.
2. Description of the Related Art
As is known, silicon carbide (SiC) is a semiconductor material with wide band gap, i.e., with a band gap energy value Eg higher than 1.1 eV, with such physical characteristics as to make it ideal for the formation of electronic switches for power applications. In the following table some physical parameters are reported of the polytypes that are more common than silicon carbide, compared with silicon (Si).
Si3C—SiC6H—SiC4H—SiCEg (eV)1.12.333.3Vsn1 × 1072.5 × 1072 × 1072 × 107μn (cm2/Vs)13501000380947εr11.89.669.79.7Ec (V/cm)2 × 105  3 × 1064 × 1063 × 106K (W/cm K)1.54.955where Eg is the energy value of the band gap, Vsn is the saturation speed of the electrons, μm is the mobility of the electrons, ∈r is the dielectric constant, Ec is the critical electric field, and k is the thermal conductivity.
From the parameters reported in such table, it is possible to deduce that power electronic devices formed on silicon carbide substrates with respect to power electronic devices formed on silicon substrates have the following advantageous characteristics:
a low output resistance in conduction being the breakdown voltage equal (due to the high value of the critical electric field Ec);
a low leakage current (thanks to the high value of the band gap energy, Eg),
high working temperature and high working frequencies (due to the high value of the thermal conductivity k and of the saturation speed Vns).
However, it is well known that to form any electronic device integrated on a silicon carbide substrate it is necessary to introduce dopant elements that produce, inside the reticular matrix of the silicon carbide substrate, some doped regions of the N type or of the P type.
In particular, nitrogen (N) and phosphorus (P) introduce donors into the reticular matrix forming doped regions of the N type, boron (B) and aluminium (Al) introduce acceptors and form doped regions of the P type.
A particularly important technological problem linked to the formation of such doped regions is that any type of dopant implanted in a silicon carbide substrate has a negligible diffusion coefficient D up to temperatures in the order of 1800° C. as described in the article “Properties of Silicon Carbide” by Gary L. Harris. In particular, at such high temperatures, nitrogen has a diffusion coefficient D in the silicon carbide equal to 5×10−12 cm2s−1, oxygen has diffusion coefficient D equal to 1.5×10−16 cm2s−1, whereas boron has a diffusion coefficient D equal to 2.5×10−13 cm2s−1.
In the silicon, instead, boron has a diffusion coefficient equal to 2.5×10−13 cm2s−1 at a temperature of about 1150° C. Thus it has about the same diffusion in silicon as it does in silicon carbide at a much more lower temperature.
The diffusion of dopant species, used for forming the doped regions necessary for the formation of power electronic devices, is thus a problematic technical factor in the case of silicon carbide substrates.
A solution in the form of a known technique for manufacturing MOSFET devices integrated on silicon carbide substrates is described in the article by J. Tan, J. A. Cooper, M. R. Melloch, IEEE vol 19, n 12, Dec. 1998, published by the University of Purdue (USA).
As shown in FIG. 1 of the present application, a MOSFET device is formed on a substrate of the 4H SiC type and of N+ type which is overlapped by a drift region of the N− type.
Above such drift region a first epitaxial layer of the N type of thickness tn and a layer of the P type forming the MOSFET transistor base region are formed in sequence. In this latter layer of the P type, a layer of the N+ type is formed. A plurality of trenches are then formed in the layer of the P type and in the first epitaxial layer of the N type until the drift region is exposed so as to insulate a portion of the layer of the N+ type, which forms a source region above the base region.
Once a selective implantation of P regions has been carried out inside the trenches, an epitaxial layer of the N type and then an oxide layer are grown on the whole device and inside the trenches. The final device is then completed with the usual metallization steps for the formation of the gate region, and of the contacts of the integrated device thus obtained.
Although advantageous in several aspects, the formation of such a device implies complex and non industrialized processes.
A technical problem underlying the present disclosure is that of devising a method for manufacturing electronic devices integrated on a semiconductor substrate wherein the doped species have a low diffusivity coefficient, such as for example silicon carbide having such characteristics as to allow the formation of such electronics devices with a high efficiency, thus overcoming the problem linked to the diffusivity of the dopant species, which limits the devices formed according to the prior art.